(a) Field of the Invention
The present invention relates to reference time correction. More particularly, the present invention relates to a method and an apparatus for correcting reference time for distributed time synchronization in a wireless communication system.
(b) Description of the Related Art
In a wireless communication system that requires distributed control such as a wireless mesh network or an AD-hoc network, in order to improve mobility of nodes and to perform flexible work, synchronization for stabilizing time and frequency offset is required when a network is initially formed or when a network is re-constituted.
In relation to time synchronization, a method of performing distributed time synchronization that may be more flexibly applied to mobility of nodes and expandability to a multi-hop is more efficient than a topology structure of a centralized method of performing time synchronization based on a reference node.
Time synchronization among nodes in a wireless mesh network may be performed using a global positioning system (GPS) based clock. However, due to vulnerability to a shadow region and a jamming signal, a method of performing distributed time synchronization without using the GPS is necessary.
In order to perform distributed time synchronization among nodes, round trip delay (RTD) values of the nodes are updated using a periodic ranging process, and a position of reference time is periodically corrected so that reference time is shared by all of neighbor nodes. As described above, in order to perform the distributed time synchronization among the nodes, it is necessary to measure the RTD values of the nodes using distributed ranging and to manage reference time of a distributed mesh network.
However, in the distributed mesh network, due to differences in frequency offsets of oscillators that generate frame reference times of the nodes, as time goes by, the frame reference times of the nodes are changed. Therefore, during a periodically performed distributed ranging operation, since the reference time that must be shared by the nodes is changed, it is not possible to measure correct RTD.